The pulvinar nucleus of the thalamus is one of the most enigmatic structures in the brain. Of thalamic nuclei, it shows the largest increase in size with evolution, keeping pace with the size of primate neocortex. Despite considerable effort, its function remains essentially completely unknown. Two related suggestions dominate current thinking about the pulvinar, and each has some experimental support. Neither, however, has been critically tested, and the present application is intended to provide this critical test of both ideas. The first idea is that the pulvinar controls the spatial location of directed attention. This idea has support - though not conclusive - from lesion studies and physiological recording studies. We plan to directly test this idea by perturbing activity in the pulvinar while recording in extrastriate cortex. We know that directed attention produces local changes in the gain of response of extrastriate neurons; activation of the pulvinar should mimic this change and reversible inactivation of the pulvinar should eliminate it. The other suggestion for the role of the pulvinar concerns the mechanism of such gain changes in sensory cortex. In particular, it has been suggested that recurrent projections between the pulvinar and cortical structures control the flow of information between cortical areas; this regulation might underlie any role in directing spatial attention. We plan to test this idea using multiple electrode recording in extrastriate cortex. Two dorsal extrastriate areas, the middle temporal (MT) and the medial superior temporal (MST) are both connected to the same subdivision of the pulvinar (Plm), and also connected with each other - MT provides a dominant source of feedforward input to MST. We will record from both structures simultaneously while again perturbing the activity in the pulvinar. If the connections with the pulvinar regulate information flow in visual cortex, we predict that such perturbation will modulate the cross-correlation of activity between MT and MST. Success on either aim will dramatically influence our thinking about the function of thalamocortical circuits.